On the Ocean Mixed Layer influence on the genesis of Mediterranean Tropical-Like cyclones

<p>The Mediterranean basin is the formation site of a vast number and type of cyclones. Among these, we can occasionally identify intense vortices showing tropical characteristics, called Tropical-Like Cyclones (TLC). Their development has been studied in several case studies, showing the influence of synoptic scale upper level forcings and mesoscale features, such as the sea surface temperature and the characteristics of the air masses on the formation area. The importance of Sea Surface Temperature (SST) consists in modulating the intense latent and sensible heat fluxes, which control the development of the TLC. For tropical cyclones, one of the most studied factors in recent years is the ocean heat content in the formation basin of these storms. We plan here to extend this analysis to TLC. Besides innovative studies with coupled atmosphere-waves-ocean numerical models, a simpler approach for investigating the sole effect of the ocean heat content consists of adopting a simplified ocean description by varying the local characteristics of the Ocean Mixed Layer (OML). In this work we use the WRF (Weather Research and Forecasting system) model, in standalone (atmospheric) mode, with 3 km grid spacing, forced with GFS-GDAL (0.25°x0.25° horizontal resolution) and SST initialization provided by the MFS-CMEMs Copernicus dataset. Two case studies of TLC are examined here, namely ROLF (06-09/11/2011) and IANOS (14-19/09/2020). The ocean is simulated with an OML approach, with SST updated at each iteration as a function of the atmospheric heat fluxes and with an average mixed layer deph (MDL) provided by the MFS-CMEMS dataset. For each TLC studied, the MDL is modified by increasing and decreasing its depth by 10 mt, 30 mt, 50 mt . The preliminary results show how the structure of the MDL influences  the intensity of the cyclone but also the structure and precipitation both in terms of quantity and location. </p>

The Effect of Atmosphere–Ocean Coupling on the Structure and Intensity of Tropical Cyclone Bejisa in the Southwest Indian Ocean

A set of numerical simulations is relied upon to evaluate the impact of air-sea interactions on the behaviour of tropical cyclone (TC) Bejisa (2014), using various configurations of the coupled ocean-atmosphere numerical system Meso-NH-NEMO. Uncoupled (SST constant) as well as 1D (use of a 1D ocean mixed layer) and 3D (full 3D ocean) coupled experiments are conducted to evaluate the impact of the oceanic response and dynamic processes, with emphasis on the simulated structure and intensity of TC Bejisa. Although the three experiments are shown to properly capture the track of the tropical cyclone, the intensity and the spatial distribution of the sea surface cooling show strong differences from one coupled experiment to another. In the 1D experiment, sea surface cooling (∼1 ∘C) is reduced by a factor 2 with respect to observations and appears restricted to the depth of the ocean mixed layer. Cooling is maximized along the right-hand side of the TC track, in apparent disagreement with satellite-derived sea surface temperature observations. In the 3D experiment, surface cooling of up to 2.5 ∘C is simulated along the left hand side of the TC track, which shows more consistency with observations both in terms of intensity and spatial structure. In-depth cooling is also shown to extend to a much deeper depth, with a secondary maximum of nearly 1.5 ∘C simulated near 250 m. With respect to the uncoupled experiment, heat fluxes are reduced from about 20% in both 1D and 3D coupling configurations. The tropical cyclone intensity in terms of occurrence of 10-m TC wind is globally reduced in both cases by about 10%. 3D-coupling tends to asymmetrize winds aloft with little impact on intensity but rather a modification of the secondary circulation, resulting in a slight change in structure.

Dark Reduction Drives Evasion of Mercury From the Ocean

Much of the surface water of the ocean is supersaturated in elemental mercury (Hg0) with respect to the atmosphere, leading to sea-to-air transfer or evasion. This flux is large, and nearly balances inputs from the atmosphere, rivers and hydrothermal vents. While the photochemical production of Hg0 from ionic and methylated mercury is reasonably well-studied and can produce Hg0 at fairly high rates, there is also abundant Hg0 in aphotic waters, indicating that other important formation pathways exist. Here, we present results of gross reduction rate measurements, depth profiles and diel cycling studies to argue that dark reduction of Hg2+ is also capable of sustaining Hg0 concentrations in the open ocean mixed layer. In locations where vertical mixing is deep enough relative to the vertical penetration of UV-B and photosynthetically active radiation (the principal forms of light involved in abiotic and biotic Hg photoreduction), dark reduction will contribute the majority of Hg0 produced in the surface ocean mixed layer. Our measurements and modeling suggest that these conditions are met nearly everywhere except at high latitudes during local summer. Furthermore, the residence time of Hg0 in the mixed layer with respect to evasion is longer than that of redox, a situation that allows dark reduction-oxidation to effectively set the steady-state ratio of Hg0 to Hg2+ in surface waters. The nature of these dark redox reactions in the ocean was not resolved by this study, but our experiments suggest a likely mechanism or mechanisms involving enzymes and/or important redox agents such as reactive oxygen species and manganese (III).

Experimental study of Wave-turbulence interaction

<p>Turbulence is ubiquitous in the topmost skin of the ocean, where it interacts with surface waves. Rapid distortion theory predicts that wave motion will increase turbulent energy, leading to a dissipation of waves [1]. Waves are believed to contribute significantly to the turbulence in the ocean mixed layer, yet field measurements are unable to validate or distinguish between models and theories [2].</p><p>In this work we study the modification of turbulence by surface waves using experimental measurements of turbulent flows in the presence of waves, in a laboratory set-up acting as a small-scale model of the water side of the ocean surface layer. Turbulent Langmuir numbers comparable to those in the ocean are achieved, ensuring scalability. Particle image velocimetry (PIV) measurements were performed in a large water channel wherein mechanically generated waves may propagate on a current. An active grid at the inlet allowed the turbulence intensity and mean flow to be tailored independently. The flow field was measured in the streamwise-vertical plane for various flow cases and waves of varying steepness and frequency. The turbulence characteristics are compared to cases without waves to study the impact of the waves on the turbulence and the results are discussed considering predictions from rapid distortion theory.</p><p> </p><p>[1] Teixeira M. and S. Belcher 2002 “On the distortion of turbulence by a progressive surface wave” Journal of Fluid Mechanics  <strong>458 </strong>229-267.</p><p>[2] D’Asaro E.A. 2014 “Turbulence in the upper-ocean mixed layer” Annual Reivew of Marine Sciences <strong>6 </strong>101-115.</p>

Correcting Net Ocean-Atmosphere CO2 Fluxes for Near-surface Temperature Deviations.

<p>We have recently shown the neglect of small temperature differences in the ocean mixed layer has led to substantial underestimates in the ocean sink for atmospheric CO<sub>2</sub> as calculated from surface pCO<sub>2</sub> observations, which we find should be increased by ~0.8 Pg Cyr-1 when globally integrated. Surface observations of ocean pCO<sub>2</sub> such as those in the SOCAT (Surface Ocean CO<sub>2</sub> Atlas, www.socat.info) are reported at a temperature typically  measured at several metres depth, but co-location of satellite estimates of the subskin surface temperature (at a few centimetres depth) differ from this, and are on average lower. In addition the top millimetre or so of the ocean is cooler than the underlying subskin because the ocean is a source of radiative and latent heat to the atmosphere. These two temperature deviations have subtly different effects on the air-sea flux of CO<sub>2</sub> as calculated by the gas exchange equation, but both result in an increase in the flux into the ocean and the combined effect is large. We are making available several datasets enabling calculation of these effects, including the regular provision of SOCAT data corrected to the subskin temperature, a climatology of the skin temperature deviation, and corrected ocean-atmosphere CO<sub>2</sub> flux estimates for the period since 1985.</p>